Every year in wintertime, scores of migrating monarch butterflies settle in Mexico at the culmination of their long southerly flight, alighting together like flickering orange fires on fir trees in a mountaintop forest. But this year marks the sixth year in the last seven that the event has diminished from a powerful blaze to a dwindling flame. Researchers say the famed migration is on a downward turn due to agricultural shifts and climate warming that some fear could spell the end of the butterflies’ yearly passage.

“The decline in the monarch population now marks a statistical long-term trend and can no longer be seen as a combination of yearly or seasonal events,” the Huffington Post reports. Conservation authorities first began annually surveying the butterflies’ shift 20 years ago, tracking their ribbon-like flight from North America and Canada to Mexico, where they overwinter before returning North once more to breed in the spring.

This past December, as usual, the World Wildlife Fund teamed up with the Mexican government and Mexican cellphone company Telcel, to estimate how many butterflies arrived in the fir forest to rest before the 2012-2013 season. Counting every individual butterfly is impossible, so researchers survey the population by estimating the hectare area encompassed by the insects once they settle and sleep, which they do by forming reef-like structures together on tree limbs. This year, the estimates show a 59% drop in monarch numbers, when compared to estimates from December 2011.

The research is not peer-reviewed, but the surveyors insist this number is the lowest since counting began two decades ago, and that it signals a major threat to the monarch. “This is not just the lowest population recorded in the 20 years for which we have records,” said monarch expert Lincoln Brower of Sweet Briar College in Virginia to the Huffington Post, “it is the continuation of a statistically significant decrease in the monarch population that began at least a decade ago.”

But where reports differ is in understanding the key threats. Essentially, the problem is three-pronged. A warming climate, the agricultural boom in corn and soy, and illegal forest logging are together working to drive monarch numbers down. Last year’s stifling heat wave that marched across the U.S. may have affected monarchs by drying up their eggs as they bred, some experts say.

Warmer than usual conditions led the insects to arrive early and to nest farther north than is typical, Chip Taylor, director of the conservation group Monarch Watch at the University of Kansas, said in an interview. The early arrival disrupted the monarchs’ breeding cycle, he said, and the hot weather dried insect eggs and lowered the nectar content of the milkweed on which they feed.

This go-to food source, the milkweed, is the keyword for the second perceived threat. The plant is the monarch’s prime nutrient source during its lifetime, but with the corn and soy farming exploits that carpet much of America’s hinterland, the milkweed is falling prey. Aside from being edged out by crops, herbicides are also now being more freely used, reports The New York Times, since crops are now genetically engineered to incorporate herbicide resistance. But herbicides still kill the unprotected droves of milkweed. “That habitat is virtually gone. We’ve lost well over 120 million acres, and probably closer to 150 million acres,” Taylor told the paper.

Mexican authorities maintain that because of this, the problem rests North of them. The country has taken steps to eliminate illegal logging in the pine forests where the monarchs alight, said WWF’s Mexico director Omar Vidal to The Guardian, meaning that the problem has its roots elsewhere. But Brower, speaking to the Guardian, contends that this “is a whitewash by the World Wildlife Fund and the Mexican government.” He added that the Mexican government and the WWF are “playing down and ignoring the continued degradation of the microclimate of the forest that is critical to the butterflies.” Illegal logging continues, he said, as does unmanaged tourism in the area, and damage to forest streams on which the monarchs depend.

Despite the bad news, experts believe that the monarchs’ resilience will buoy up the population enough to keep it from dwindling too far—for the time being. But if the population crosses a tipping point, said Taylor to The New York Times, the prospects are dim: Future cold weather or high temperatures could send the butterflies flying toward extinction.